Advanced Control Circuit for Switched-Mode DC-DC Converter

ABSTRACT

A voltage converter (FIG.  4 ) for a power supply circuit is disclosed. The voltage converter comprises a control circuit ( 400 ) coupled to receive an enable (EN) signal. The control circuit produces a first control signal (PWM) to provide a load current (I L ) in response to the enable signal. A sample and hold circuit ( 408 ) is arranged to produce a third control signal (CSP) to emulate the load current and a fourth control signal (CSN′) to sample and hold value of the third control signal. A comparator circuit ( 416 ) is arranged to compare the third and fourth control signals and produce the enable signal in response to a result of the comparison.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional of U.S. patent application Ser.No. 14/330,102, filed Jul. 14, 2014, which is a continuation-in-part ofU.S. patent application Ser. No. 13/774,496, filed Feb. 22, 2013, whichare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to closed loop voltagecontrol with adaptive on time control schemes. A preferred embodiment ofthe invention is intended for use in a DC-DC switching regulatorcircuit, but the circuit may also be used in other applications thatrequire closed loop voltage regulation.

Referring to FIGS. 1 and 2A, there is a DC-DC switching regulatorcircuit of the prior art as disclosed by Tateishi et al. in U.S. Pat.No. 7,595,624. The switching regulator includes a pulse-width modulation(PWM) controller 52 configured to alternately activate 10 a high-sidetransistor 54 and a low-side transistor 56 as shown at FIG. 1. The PWMcontroller 52 controls the duty cycle of pulses provided to bothtransistors 54 and 56. Transistor 54 has a drain terminal that isconnected to a supply voltage source V_(DD). Transistor 56 has a sourceterminal connected to ground. Transistors 54 and 56 have a commonterminal that produces switching voltage V_(SW). An inductor 60 isinterconnected between the common terminal and an output terminalproducing a voltage V_(OUT). A freewheeling diode 62 is interconnectedbetween one terminal of inductor 60 and ground.

During an on-time, controller 52 activates transistor 54 whiletransistor 56 remains inactive. When transistor 54 is on, voltage V_(SW)increases to approximately V_(DD). Consequently, current throughinductor 60 increases. During an off-time, the controller 52 deactivatestransistor 54 and activates transistor 56. Consequently, voltage V_(SW)decreases to approximately ground. Current through inductor 60, however,tends to remain unchanged. Thus, the voltage V_(SW) becomes negativerelative to ground so that inductor current is supplied through thefreewheeling diode 62. Accordingly, the switching regulator operates tomaintain the current flow through inductor 60, thus providing an outputvoltage V_(OUT) across a load capacitor 64 and a load resistor 66.

The switching regulator also includes a PWM comparison circuit 68-1 todetermine the off-time relative and the on-time of transistor 54. Theoutput voltage V_(OUT) is applied to the PWM comparison circuit througha voltage divider 70 that includes resistors R₁ and R₂. The voltagedivider provides a feedback voltage V_(FB) as an output. The PWMcomparison circuit includes a ramp signal generator 72-1 configured togenerate a ramp signal, preferably by alternately charging ordischarging a capacitor. The feedback voltage V_(FB) and the ramp signalare added together by adder 74-1. A resultant modified feedback voltageV_(FB2) is provided to a comparator or error amplifier 76, whichcompares the modified feedback voltage V_(FB2) and the reference voltageV_(REF). The comparator 76 thus provides an output to the PWM controller52 to switch between the on-time and the off-time. In this manner,on-time (T_(ON)) is approximately equal to the switching time(T_(SW)=T_(ON)+T_(OFF)) multiplied by V_(OUT)/V_(DD), where theswitching regulator frequency is equal to 1/T_(SW).

As the operating frequency of the switching regulator circuit of FIG. 2Aincreases, several problems limit circuit efficiency. Due to noise orother effects, reference voltage V_(REF) can be subject to error suchthat output voltage V_(OUT) is compared to V_(REF′), thereby producingerror voltage V_(E) as shown at FIG. 2A. This error voltage produces anext on-time error T₀ and a premature output ripple voltage 12. Tateishiet al. disclose an improvement as shown at FIG. 2B. There, the maximumfeedback voltage V_(FB) is added to reference voltage V_(R) to producemodified feedback voltage V_(FB2). The relatively steeper slope ofV_(FB2) decreases the uncertainty on-time from T_(UNC1) to T_(UNC2).

The circuit of FIG. 2A was subsequently improved with the addition ofthe current sense circuit of FIG. 3A connected in parallel with inductor60 as disclosed by Texas Instruments Inc., Application ReportSLVA453-February 2011. The current sense circuit includesseries-connected resistor 300 and capacitor 302 in parallel withinductor 60 and produces positive current sense signal CSP.Series-connected resistor 304 and capacitor 306 are connected inparallel with capacitor 302 and produce negative current sense signalCSN.

Referring now to FIG. 3B, there is a timing diagram showing operation ofthe current sense circuit of FIG. 3A in continuous current mode (CCM).Here, CCM means that either transistor 54 or 56 (FIG. 2A) is always on.At time t1 transistor 54 is activated and voltage V_(SW) goes high.Responsively, voltage CSP emulates the inductor current I_(L) untiltransistor 54 is turned off and transistor 56 is turned on. At time t2,CSP again emulates inductor current I_(L) and goes low to a valley attime t3. CSN, however, has an intermediate value between the peak andvalley of CSP.

Referring next to FIG. 3C, there is a timing diagram showing operationof the current sense circuit of FIG. 3A in discontinuous current mode(DCM). Here, DCM means that either transistor 54 or 56 (FIG. 2A) is onor both are off. At time t1 transistor 54 is activated and voltageV_(SW) goes high. Voltage CSP again emulates the inductor current I_(L)until transistor 54 is turned off and transistor 56 is turned on. Attime t2, CSP again emulates inductor current I_(L) and decreases untiltime t3 when transistor 56 is turned off. Between times t3 and t4, bothtransistors 54 and 56 are off, inductor current IL is zero, and CSP andCSN vary as capacitors 302 and 306 discharge. The offset of CSN in CCMand DCM, is not constant and is, therefore, difficult to cancel whencompared to CSP. Various embodiments of the present invention aredirected to solving these problems and improving operation of theswitching regulator.

BRIEF SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, a voltage converterfor a power supply is disclosed. The voltage converter includes acontrol circuit coupled to receive an enable signal. The control circuitproduces a first control signal to provide a load current in response tothe enable signal. A sample and hold circuit is arranged to produce athird control signal to emulate the load current and a fourth controlsignal to sample and hold a value of the third control signal. Acomparator circuit is arranged to compare the third and fourth controlsignals and produce the enable signal in response to a result of thecomparison.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a timing diagram showing operation of the DC-DC voltageregulator circuit of FIG. 2A;

FIG. 2A is a DC-DC voltage regulator circuit of the prior art;

FIG. 2B timing diagram showing an improvement of the circuit of FIG. 2A;

FIG. 3A is a current sense circuit of the prior art added as animprovement to the circuit of FIG. 2A;

FIGS. 3B and 3C are timing diagrams showing operation of the circuit ofFIG. 3A in continuous current mode (CCM) and discontinuous current mode(DCM), respectively;

FIG. 4 is a DC-DC voltage regulator circuit of the present invention;

FIG. 5A is a current sample and hold (CSH) circuit of the presentinvention;

FIG. 5B is a clock generator circuit for the current sample and holdcircuit of FIG. 5A;

FIGS. 5C and 5D are timing diagrams showing operation of the circuit of

FIG. 4 in continuous current mode (CCM) and discontinuous current mode(DCM), respectively;

FIG. 6 is a schematic diagram of the 4-input comparator circuit of FIG.4; and

FIG. 7 is a schematic diagram of the PWM control circuit of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention provide significantadvantages over DC-DC voltage converters of the prior art as will becomeevident from the following detailed description.

Referring to FIG. 4, there is a DC-DC voltage regulator circuit of thepresent invention. Here and in the following discussion, the samereference numerals are used in various drawing figures to indicate thesame circuit elements. The voltage regulator includes a pulse widthmodulation (PWM) control circuit 400 that produces control signal PWMand complementary control signal /PWM. In some embodiments of thepresent invention, complementary control signal /PWM may be omitted.N-channel transistor 402 has a current path coupled between supplyvoltage V_(DD) and voltage terminal V_(SW) controlled by PWM. N-channeltransistor 404 has a current path coupled between voltage terminalV_(SW) and reference supply voltage terminal V_(SS). The current path ofn-channel transistor 404 is controlled by /PWM. Inductor 406 is coupledbetween voltage terminal V_(SW) and output terminal V_(OUT) to supplyload current I_(L) to load capacitance 410 and load resistance 412. Acurrent sample and hold circuit 408 is coupled in parallel with inductor406 and produces control signals CSP and CSN′ as will be explained indetail.

Output voltage V_(OUT) is fed back to a voltage divider formed byresistors R₁ and R₂ to produce feedback voltage V_(FB) at their commonterminal. Feedback voltage V_(FB) is compared with reference voltageV_(REF) and CSP is compared with CSN′ by comparator circuit 416 toproduce enable signal EN. The comparison of V_(FB) with V_(REF) has again factor a (414) relative to the comparison of CSP and CSN′. In apreferred embodiment of the present invention, gain factor α has a valueof 4 as determined by relative conductivity of comparator transistors.

Turning now to FIG. 5A, current sample and hold circuit 408 will bedescribed in detail. Resistor 500 and capacitor 502 are connected inseries between voltage terminal V_(SW) and output terminal V_(OUT) toform an RC filter. Control signal CSP emulates the current throughinductor 406. Here, emulate means minimum and maximum values of CSPoccur at substantially the same time as minimum and maximum values ofI_(L), respectively. Moreover, CSP increases as I_(L) increases. A firstbuffer (BUF1) applies CSP to resistor 512 and to switches controlled byclock signals Φ₁ and Φ₂. Here and in the following discussion, switchesare used by way of explanation. In preferred embodiments of the presentinvention these switches may be bipolar transistors, MOS transistors,CMOS transmission gates, or other devices that are well known in theart. A switch controlled by clock signal Φ₃ selectively connectsresistor 512 to the input terminal of a second buffer (BUF2). Capacitor506 is coupled to receive and store buffered signal CSP (VC1) when theswitch controlled by clock signal Φ₁ is closed. Likewise, capacitor 508is coupled to receive and store buffered signal CSP (VC2) when theswitch controlled by clock signal Φ₂ is closed. Clock signals Φ₁ and t₂are alternately activated to apply one of VC1 and VC2 to the inputterminal of buffer BUF2 when Φ₂ and Φ₁ are activated, respectively.Buffer BUF2 applies the respective sampled CSP (VC1 or VC2) to capacitor504 via resistor 510 where it is stored as CSN′.

Referring now to FIG. 5B, operation of the circuit to generate clocksignals Φ₁, Φ₂, and Φ₃ for the circuit of FIG. 5A will be described indetail. Pull up (PU) and pull down (PD) control signals are generated bythe PWM control circuit of FIG. 7 and correspond to control signals PWMand /PWM as will be described in detail. Delay flip flop 520 is coupledto receive control signal PU at a clock terminal and produces a highlevel of clock signal Φ₁ at a first time. Inverter 522 responsivelyproduces a complementary low level of clock signal Φ₂ at the first time.The complementary output (/Q) of delay flip flop 520 is coupled to inputterminal D so that a next high level of control signal PU at the clockterminal and produces a low level of clock signal Φ₁ at a second time.Inverter 522 responsively produces a complementary high level of clocksignal Φ₂ at the second time.

In a discontinuous operating mode (DCM), control signals PU and PD areboth low at the same time. NOR gate 524 produces a high level signal atthe input of a delay inverter formed by p-channel transistor 526,n-channel transistor 528, and an intermediate resistor. The output ofthe delay inverter is coupled to capacitor 532 to provide an RC outputdelay. Inverter 530 is coupled to receive the output signal from thedelay inverter and produce a high level of clock signal Φ₃ while PU andPD are both low. The delay inverter and RC elements are preferablydesigned to provide adequate time for PU and PD to settle so that CSPand CSN′ are approximately equal.

Turning now to FIG. 5C, continuous current mode (CCM) of the circuits ofFIGS. 5A and 5B will be described in detail. At time t1, signal V_(SW)(FIG. 4) goes high in response to control signal PU and correspondingsignal PWM. Inductor current I_(L) increases to a peak value at time t2while V_(SW) is high. Clock signal Φ₁ also goes high at time t1 whileclock signal Φ₂ remains low. CSP emulates inductor current I_(L) andattains a peak value at time t2. Since clock signal Φ₁ is high, VC1tracks CSP. CSN′ and VC2 remain low at a previously sampled value of CSPstored on capacitor 508. At time t2, PU and PWM go low and PD and /PWMgo high. Responsively, load current I_(L) decreases to a minimum valueat time t3. CSP again emulates inductor current I_(L) and attains aminimum value at time t3. Since clock signal Φ₁ is high until time t3,VC1 tracks CSP. At time t3, clock signals Φ₁ and 02 go low and high,respectively. The low level of Φ₁ stores a minimum value of CSP oncapacitor 506 and disconnects capacitor 508 from BUF2. The high level ofΦ₂ connects capacitor 506 to BUF2 and stores a new minimum value of CSP(VC2) on capacitor 508.

At time t3, signal V_(SW) goes high again in response to control signalPU and corresponding signal PWM. Inductor current I_(L) increases to apeak value at time t4 while V_(SW) is high. Clock signal Φ₂ goes high attime t3 while clock signal Φ₁ remains low. CSP emulates inductor currentI_(L) and attains a peak value at time t4. Since clock signal Φ₂ ishigh, VC2 tracks CSP. CSN′ and VC1 remain low at a previously sampledvalue of CSP stored on capacitor 506. At time t4, PU and PWM go low andPD and /PWM go high. Responsively, load current I_(L) decreases to aminimum value at time t5. CSP again emulates inductor current I_(L) andattains a minimum value at time t5. Since clock signal Φ₂ is high untiltime t5, VC2 tracks CSP. At time t5, clock signals Φ₁ and Φ₂ go high andlow, respectively. The low level of Φ₂ stores a new minimum value of CSP(VC2) on capacitor 508 and disconnects capacitor 506 from BUF2. The highlevel of Φ₁ connects capacitor 508 to BUF2 and stores a new minimumvalue of CSP (VC1) on capacitor 506. The foregoing sequence continues sothat one of transistor 402 or 404 is on and the other is off

Referring next FIG. 5D, discontinuous current mode (DCM) of the circuitsof FIGS. 5A and 5B will be described in detail. At time t1, signalV_(SW) (FIG. 4) goes high in response to control signal PU andcorresponding signal PWM. Inductor current I_(L) increases to a peakvalue at time t2 while V_(SW) is high. Clock signal Φ₁ also goes high attime t1 while clock signals Φ₂ and Φ₃ remain low. CSP emulates inductorcurrent I_(L) and attains a peak value at time t2. Since clock signal Φ₁is high, VC1 tracks CSP. CSN′ and VC2 remain low at a previously sampledvalue of CSP stored on capacitor 508. At time t2, PU and PWM go low andPD and /PWM go high. Responsively, load current I_(L) decreases to aminimum value at time t3. CSP again emulates inductor current I_(L) andattains a minimum value at time t3. Since clock signal Φ₁ is high, VC1tracks CSP. At time t3, PD and /PWM go low and PU and PWM remain low.Clock signals Φ₁ and Φ₂ remain high and low, respectively. The low levelof PU and PD at the input terminals of NOR gate 524 produces a highlevel output signal that is buffered by the delay inverter (526 and 528)and inverter 530 to produce a high level of clock signal Φ₃. From timet3 until time t4, inductor current remains near zero and V_(SW) isapproximately equal to \T_(our). Clock signal Φ₃ remains high andconnects the output of BUF1 to the input of BUF2 via resistor 512.Between t3 and t4, therefore, the present value of CSP replaces thepreviously sampled value (VC2) on capacitor 508. Responsively, BUF2produces CSN′ as the present value of CSP. This advantageously avoidsleakage of a stored CSP sample on capacitor 508 and maintains CSN′ equalto CSP for any time period from t3 to t4.

At time t4, signal V_(SW) goes high again in response to control signalPU and corresponding signal PWM. The high level of PU drives clocksignal Φ₃ low to disconnect the output of BUF1 from the input of BUF2.Inductor current I_(L) increases to a peak value at time t5 while V_(SW)is high. Clock signal Φ₂ goes high at time t4 and clock signal Φ₁ goeslow. CSP emulates inductor current I_(L) and attains a peak value attime t5. Since clock signal Φ₂ is high, VC2 tracks CSP. CSN′ and VC1remain low at a previously sampled value of CSP stored on capacitor 506.At time t5, PU and PWM go low and PD and /PWM go high. Responsively,load current I_(L) decreases to a minimum value at time t6. CSP againemulates inductor current I_(L) and attains a minimum value at time t6.Since clock signal Φ₂ is high until time t6, VC2 tracks CSP. Theforegoing sequence may continue in DCM or transition to CCM in responseto varying load conditions.

Referring now to FIG. 6, the 4-input comparator circuit 416 of FIG. 4will be described in detail. In a preferred embodiment of the presentinvention, the 4-input comparator circuit includes five comparators600-608. Comparator 600 compares reference voltage V_(REF) to feedbackvoltage V_(FB). Comparator 602 compares CSN′ to CSP. As previouslydiscussed, comparator 600 has a gain factor a greater than the gain ofcomparator 602. This gain factor is achieved by increasing theconductivity of transistors MP1 and MP2 with respect to transistors MP3and MP4. Series connected comparators 604 and 606 amplify the combineddifference voltage developed by comparators 600 and 602. Comparator 608further amplifies the difference voltage from comparator 606 and appliesa single output signal to inverter 610. Inverter 610 buffers the singleoutput and produces enable signal EN, which is applied to PWM controlcircuit 400.

In operation, enable signal EN goes high when the common drain terminal(A) of transistors MP2 and MP4 is positive with respect to the commondrain terminal (B) of transistors MP1 and MP3. This condition may occurwhen feedback voltage V_(FB) is less than reference voltage V_(REF) orwhen current sense signal CSP is less than current sense signal CSN′. Alow-to-high transition of enable signal EN initiates a new on-time pulsein the PWM control circuit of FIG. 7 as will be explained in detail.

Referring to FIG. 7, there is a simplified schematic diagram of the PWMcontrol circuit of FIG. 4. The PWM control circuit includes four majorparts that will be discussed separately as well as control logic. First,the on-time timer section includes comparator 708, current source 710,capacitor 712, and n-channel transistor 714. N-channel transistor 714initially discharges capacitor 712 while current source 710 is off. Atthe beginning of an on-time pulse, n-channel transistor 714 is turnedoff and current source 710 is turned on. Duration of the on-time pulseis determined by the time required for current source 710 to chargecapacitor 712. Comparator 708 compares the voltage on capacitor 712 withoutput voltage V_(OUT). When the voltage on capacitor 712 exceeds outputvoltage V_(OUT), comparator 708 produces a high level output signal atthe reset terminal of SR flip flop 702. This resets SR flip flop 702 andproduces a low output signal at the Q terminal, thereby terminating theon-time pulse.

Second, the off-time timer section includes comparator 722, referencevoltage source 724, current source 716, capacitor 718, and n-channeltransistor 720. N-channel transistor 720 initially discharges capacitor718 while current source 716 is off. At the beginning of an off-timepulse, n-channel transistor 720 is turned off and current source 716 isturned on. Duration of the off-time pulse is determined by the timerequired for current source 716 to charge capacitor 718. Comparator 722compares the voltage on capacitor 718 with reference voltage source 724.When the voltage on capacitor 718 exceeds reference voltage source 724,comparator 722 produces a high level output signal at one input terminalof OR gate 728. The other input of OR gate 728 is still low asdetermined by the Q output of SR flip flop 702 in the previous on-timeperiod. The high level output from OR gate 728 is applied to one inputof AND gate 700. However, the next on-time pulse will not begin untilenable signal EN goes high. The high level output from OR gate 728 isapplied to inverter 730 to drive the S terminal of SR flip flop 734 low.The high level output from OR gate 728 is also applied to AND gate 732to drive the R terminal of SR flip flop 734 high. The high level of thereset terminal of SR flip flop 734 produces a low output signal at the Qterminal, thereby terminating the off-time pulse.

Third, the zero crossing section includes comparator 740 and switchSW_(ZC). The Q output of SR flip flop 734 is high during an off-timepulse, thereby producing a high level of PD and /PWM so that n-channeltransistor 404 (FIG. 4) is on. The high level of the Q output of SR flipflop 734 activates switch SW_(ZC) so that comparator 740 comparesvoltage V_(SW) to reference voltage V_(SS) or ground. If voltage V_(SW)goes negative with respect to ground during the off-time period,comparator 740 produces a low level output at an input of OR gate 726.Together with the low level from the Q terminal of SR flip flop 702, ORgate 726 produces a low level at an input of AND gate 732 to produce alow level at the R input of SR flip flop 734. The low level of the Rinput of SR flip flop 734 prevents termination of the off-time pulseeven if the off-time timer has expired. Thus, n-channel transistor 404remains on while voltage V_(SW) is negative with respect to ground toprevent any significant undershoot that might induce minority carrierinjection.

Fourth, the cross-conduction control section includes AND gates 704 and736, buffers 706 and 738, comparators 742 and 744, and reference voltagesources 746 and 748. The cross-conduction control section assures thatn-channel transistors 402 and 404 are never on at the same time. Duringan on-time period, for example, AND gate 704 produces a high levelsignal PU. Buffer 706 buffers this signal to provide a high level of PWMat the gate of n-channel transistor 402 as well as at one input ofcomparator 742. While the level of PWM exceeds reference voltage 746,comparator 742 produces a low level output that turns off n-channeltransistor 714 and produces a low level signal PD and buffered signal/PWM. Thus, PD and /PWM remain low while PWM is greater than referencevoltage 746. Likewise, during an off-time period AND gate 736 produces ahigh level signal PD. Buffer 738 buffers this signal to provide a highlevel of /PWM at the gate of n-channel transistor 404 as well as at oneinput of comparator 744. While the level of /PWM exceeds referencevoltage 748, comparator 744 produces a low level output which, in turn,produces a low level signal PU and buffered signal PWM. Thus, PU and PWMremain low while /PWM is greater than reference voltage 748.

Still further, while numerous examples have thus been provided, oneskilled in the art should recognize that various modifications,substitutions, or alterations may be made to the described embodimentswhile still falling within the inventive scope as defined by thefollowing claims. For example, although the previous on-time andoff-time circuits disclose current ramps having a positive slope,alternative embodiments of the present invention may also employ currentramps having a negative slope or a combination of positive and negativeslopes. Moreover, although embodiments of the present invention disclosea positive power supply voltage, other embodiments of the presentinvention may also be directed to a negative power supply voltage. Othercombinations will be readily apparent to one of ordinary skill in theart having access to the instant specification.

What is claimed is:
 1. A voltage converter, comprising: a control circuit coupled to receive an enable signal, the control circuit producing a first control signal to provide a load current to an output terminal in response to the enable signal; a sample and hold circuit arranged to produce a third control signal to emulate the load current and a fourth control signal to sample and hold a value of the third control signal; and a comparator circuit arranged to compare the third and fourth control signals and produce the enable signal in response to a result of the comparison.
 2. A voltage converter as in claim 1, comprising: a load capacitor coupled to the output terminal; and an inductor arranged to conduct the load current to the load capacitor in response to the first control signal.
 3. A voltage converter as in claim 1, wherein the control circuit produces a second control signal that is complementary to the first control signal.
 4. A voltage converter as in claim 1, wherein the control circuit produces a second control signal having a low level while the first control signal has a high level, and wherein the third and fourth control signals are equal when the first and second control signals both have a low level.
 5. A voltage converter as in claim 1, wherein the value of the fourth control signal is a sample of a minimum value of the third control signal.
 6. A voltage converter as in claim 1, wherein the third control signal alternately has minimum and maximum values, and wherein the fourth control signal is a successive sample of each minimum value of the third control signal.
 7. A voltage converter as in claim 1, comprising: a current source; a capacitor coupled to receive a current from the current source; and a comparator having a first input terminal coupled to the capacitor and having a second input terminal coupled to the output terminal, wherein a duration of the first control signal is determined by an output signal from the comparator.
 8. A power supply, comprising: a first output transistor having a first control terminal and having a current path coupled between a supply voltage and an output terminal; a second output transistor having a second control terminal and having a current path coupled between the output terminal and a reference terminal; a control circuit coupled to receive an enable signal, the control circuit producing a first control signal at the first control terminal in response to the enable signal, and producing a second control signal at the second control terminal; a sample and hold circuit arranged to produce a third control signal to emulate the load current and a fourth control signal to sample and hold a value of the third control signal; and a comparator circuit arranged to compare the third and fourth control signals and produce the enable signal in response to a result of the comparison.
 9. A power supply as in claim 8, comprising: a load capacitor coupled to the output terminal; and an inductor arranged to conduct the load current to the load capacitor in response to the first control signal.
 10. A power supply as in claim 8, wherein the first and second control signals are complementary in a continuous mode of operation.
 11. A power supply as in claim 8, wherein the first and second control signals are both low at a first time in a discontinuous mode of operation.
 12. A power supply as in claim 8, wherein the value of the fourth control signal is a sample of a minimum value of the third control signal.
 13. A power supply as in claim 8, wherein the value of the fourth control signal is a sample of a maximum value of the third control signal.
 14. A power supply as in claim 8, wherein the third control signal alternately has minimum and maximum values, and wherein the fourth control signal is a successive sample of each minimum or maximum value of the third control signal.
 15. A method of regulating an output voltage, comprising: producing an on-time signal having a first duration in response to an enable signal; producing a load current in response to the on-time signal; producing a first voltage signal emulating the load current; producing a second voltage signal having a sampled value of the first voltage signal; comparing the first and second voltage signals; and producing the enable signal in response to a result of the step of comparing.
 16. A method as in claim 15, comprising: producing an off-time signal having a second duration after the step of producing the on-time signal.
 17. A method as in claim 15, wherein the sampled value is a minimum value of the first voltage signal.
 18. A method as in claim 15, wherein the first voltage signal comprises a sequence of alternating minimum and maximum values, and wherein the second voltage signal is a sample of each successive minimum value of the first voltage signal.
 19. A method as in claim 15, wherein the first voltage signal comprises a sequence of alternating minimum and maximum values, and wherein the second voltage signal is a sample of each successive maximum value of the first voltage signal.
 20. A method as in claim 15, wherein the step of comparing comprises comparing a feedback signal from the output voltage to a reference voltage. 